Aromatic hydrocarbons, particularly benzene, toluene, ethylbenzene and xylenes, are important commodity chemicals in the petrochemical industry. Currently, aromatics are most frequently produced from petroleum-based feedstocks by a variety of processes, including catalytic reforming and catalytic cracking. However, as the world supplies of petroleum feedstocks decrease, there is a growing need to find alternative sources of aromatic hydrocarbons.
One possible alternative source of aromatic hydrocarbons is methane, which is the major constituent of natural gas and biogas. World reserves of natural gas are constantly being upgraded and more natural gas is currently being discovered than oil. Because of the problems associated with transportation of large volumes of natural gas, most of the natural gas produced along with oil, particularly at remote places, is flared and wasted. Hence the conversion of alkanes contained in natural gas directly to higher hydrocarbons, such as aromatics, is an attractive method of upgrading natural gas, providing the attendant technical difficulties can be overcome.
A large majority of the processes currently proposed for converting methane to liquid hydrocarbons involve initial conversion of the methane to synthesis gas, a blend of H2 and CO. However, production of synthesis gas is capital and energy intensive and hence routes that do not require synthesis gas generation are preferred.
A number of alternative processes have been proposed for directly converting methane to higher hydrocarbons. One such process involves catalytic oxidative coupling of methane to olefins followed by the catalytic conversion of the olefins to liquid hydrocarbons, including aromatic hydrocarbons. For example, U.S. Pat. No. 5,336,825 discloses a two-step process for the oxidative conversion of methane to gasoline range hydrocarbons comprising aromatic hydrocarbons. In the first step, methane is converted to ethylene and minor amounts of C3 and C4 olefins in the presence of free oxygen using a rare earth metal promoted alkaline earth metal oxide catalyst at a temperature between 500° C. and 1000° C. The ethylene and higher olefins formed in the first step are then converted to gasoline range liquid hydrocarbons over an acidic solid catalyst containing a high silica pentasil zeolite.
However, oxidative coupling methods suffer from the problems that they involve highly exothermic and potentially hazardous methane combustion reactions and they generate large quantities of environmentally sensitive carbon oxides.
A potentially attractive route for upgrading methane directly into higher hydrocarbons, particularly ethylene, benzene and naphthalene, is dehydroaromatization or reductive coupling. This process typically involves contacting the methane with a catalyst comprising a metal, such as rhenium, tungsten or molybdenum, supported on a zeolite, such as ZSM-5, at high temperature, such as 600° C. to 1000° C. Frequently, the catalytically active species of the metal is the zero valent elemental form or a carbide or oxycarbide.
For example, U.S. Pat. No. 4,727,206 discloses a process for producing liquids rich in aromatic hydrocarbons by contacting methane at a temperature between 600° C. and 800° C. in the absence of oxygen with a catalyst composition comprising an aluminosilicate having a silica to alumina molar ratio of at least 5:1, said aluminosilicate being loaded with (i) gallium or a compound thereof and (ii) a metal or a compound thereof from Group VIIB of the Periodic Table.
In addition, U.S. Pat. No. 5,026,937 discloses a process for the aromatization of methane which comprises the steps of passing a feed stream, which comprises over 0.5 mole % hydrogen and 50 mole % methane, into a reaction zone having at least one bed of solid catalyst comprising ZSM-5, gallium and phosphorus-containing alumina at conversion conditions which include a temperature of 550° C. to 750° C., a pressure less than 10 atmospheres absolute (1000 kPaa) and a gas hourly space velocity of 400 to 7,500 hr−1.
Moreover, U.S. Pat. Nos. 6,239,057 and 6,426,442 disclose a process for producing higher carbon number hydrocarbons, e.g., benzene, from low carbon number hydrocarbons, such as methane, by contacting the latter with a catalyst comprising a porous support, such as ZSM-5, which has dispersed thereon rhenium and a promoter metal such as iron, cobalt, vanadium, manganese, molybdenum, tungsten or a mixture thereof. After impregnation of the support with the rhenium and promoter metal, the catalyst is activated by treatment with hydrogen and/or methane at a temperature of about 100° C. to about 800° C. for a time of about 0.5 hr. to about 100 hr. The addition of CO or CO2 to the methane feed is said to increase the yield of benzene and the stability of the catalyst.
However, the successful application of reductive coupling to produce aromatics on a commercial scale requires the solution of a number of serious technical challenges. In particular, the process is highly endothermic, thereby requiring large amounts of heat to be supplied to the reaction. Moreover, the process must be conducted at very high temperatures, typically 800° C. to 1000° C., to ensure reasonable rates of methane conversion. This in turn leads to significant metallurgical challenges in formulating the surfaces of the reactor required to withstand the highest process temperatures and process gases.
Thus, when exposed to hydrocarbons at high temperature, many metals and metal alloys tend to form internal carbides that can cause degradation of their mechanical properties as a result of changes in the local composition of the metal matrix. In addition, carburization can lead to the formation of metastable surface carbides that decompose on subsequent coke deposition and result in the phenomenon of metal dusting, where the surface of the metal disintegrates into powdery carbon and metal particles. The loss of metal results in the formation of pits or holes in the surface of the affected metal component and rapid thinning of the component walls. Moreover, the released metal particles can act as catalysts for the conversion of the hydrocarbon feed into undesirable coke.
Conventional techniques for mitigating carburization in high temperature reactor components include maintaining a protective oxide layer, such as chromium oxide, on the component surfaces by ensuring sufficient oxygen partial pressure in the reactor, typically by the addition of water, and introducing sulfur into the reactor so as to decrease the tendency for metal/hydrocarbon reactions. However, in the reductive coupling of methane to higher hydrocarbons, these approaches are typically unavailable or may have undesirable consequences. The addition of water or other oxygen source to the reactor would tend to convert the methane feed to unwanted carbon oxides, whereas sulfur tends to poison the catalysts generally employed to facilitate reductive coupling reactions and/or downstream conversion steps.
For example, conventional metallurgy used for steam cracking service (e.g., Incoloy® alloy 803) is found to undergo rapid carburization under methane reductive coupling conditions. Alloy 803 has a carbon uptake (grams of carbon absorbed per m2 of exposed surface area) in excess of 80 g/m2 after 168 hours of exposure at 900° C. under 50:50 vol % CH4—H2 mixture, indicating rapid formation of internal carbides. Although the temperature for methane reductive coupling is comparable to steam cracking, it has significantly lower oxygen partial pressure (thereby rendering chromium oxide unstable), leading to rapid carburization of unprotected steam cracking alloy.
In addition to carburization of bulk alloys to form internal carbides, surface reaction of hydrocarbons with metal surfaces and/or reaction of hydrocarbons with metal fines released from alloy surface due to metal dusting can lead to significant coking at exposed metal surfaces under reductive coupling process conditions. Conventional alloys and pure metals (such as nickel, cobalt, and iron) show significant coke formation on metal surfaces after exposure to 50:50 vol % CH4—H2 mixture at 900° C.
An article entitled “Alloy Solutions to Metal Dusting Problems in the Petrochemical Industry” by Baker et al., Special Metals Corporation, Huntington, W. Va. discusses the effect of metal alloy composition on resistance to metal dusting experienced in the steam reforming of methane to produce synthesis gas. The article concludes that nickel based alloys that contain high levels of scale-forming and carbide-forming elements are particularly resistant to corrosion by synthesis gas at temperatures of 400° C. to 800° C.
According to the present invention it has now been found that the problem of carburization of reactor surfaces in reductive coupling reactions can be mitigated by producing the surfaces from a refractory alloy containing at least 2 weight % of at least one of aluminum, magnesium or cerium and/or from a refractory metal or alloy capable of forming a stable, continuous carbide layer under reductive coupling conditions. In addition, it is found that by providing such surfaces with a refractory abrasion-resistant coating, which need not itself be resistant to carbon ingress, the surfaces can be used in reactor internals, such as gas distributors, slide valves and cyclones, that are exposed high velocity gas, moving catalysts particles and other highly erosive environments.